1. Field of the Invention
The invention relates to a heating plate for workpieces, comprising a heatable body which on a first side has a contact surface for the respective workpiece.
2. The Prior Art
Heating plates of this type are, for example, used for laminating workpieces and in this connection, in particular, used in laminating presses for producing photovoltaic modules. During lamination, a thin, generally film-like layer is attached to a carrier material. In many cases, for example during the production of photovoltaic modules, it is necessary to carry out the laminating process at raised temperatures (hot lamination). In this connection, generally the workpiece to be processed (i.e. the carrier material covered by the layer to be attached) is arranged on a contact surface for the workpiece configured on a heating plate, heated to a predetermined temperature and subsequently pressed. For the laminating process, it is important that the temperature distribution on the contact surface is as uniform as possible for the respective workpiece.
A first type of such a heating plate generally has rod-shaped heating devices, which are directly attached to a heating plate body. As the heat is discharged via thermal conduction to the heating plate body, a contact which is as efficient as possible between the respective heat sources of the heating devices and the heating plate body is required. This is achieved by inserting the heating devices in precisely manufactured grooves, the heating devices being able to be clamped and the thermal conduction being able to be additionally improved by applying heat conducting paste. However, even under optimal conditions, it is also difficult to achieve an uniform temperature distribution over a large surface. This is a drawback with regard to the lamination of large-surfaced workpieces, for example for producing photovoltaic modules, which may have a surface area of more than 1 m2.
In a second type of heating plate, as is disclosed for example in EP1340611 A2, cavities are provided through which a liquid heat transfer medium flows, which is used for heating and/or cooling the respective heating plate. The heating, the cooling and the temperature adjustment of the heat transfer medium takes place in this case outside the heating plate by means of a heating circuit, which comprises a heating unit, and a cooling circuit, which has at least one cooler. The heating unit and the respective cooler for the heat transfer medium are in this case connected via various conduits to the respective cavities of the heating plate. As the heat transfer medium discharges and/or absorbs a great deal of thermal energy when flowing through the cavities of the heating plate, which are generally relatively long, it is also difficult with this type of heating plate to keep the temperature constant for the respective workpiece over the entire contact surface.
In EP1517585 B1 a further type of heating plate is disclosed, in which a heating fluid is used for heating, which flows through cavities formed in the respective heating plate. The heating fluid is in this case heated by heating elements, which are located within the cavities and around which the heating fluid flows. This solution is costly, as the heating elements have to be electrically insulated and sealed against the fluid. Moreover, all heating elements are provided with spacers which ensure a centering of the heating elements in the respective cavities, in order to prevent a direct contact of the heating elements with the walls of the cavities. In this case it is difficult to accommodate such heating elements in cavities where there is a complicated arrangement of cavity walls, for example in a system consisting of a plurality of cavities which are angled relative to one another, or in cavities which are repeatedly branched. Accordingly it is possible that the aforementioned heating elements are not able to be installed in cavities where there is a complicated arrangement of cavity walls, or possibly only at considerable cost. This is a drawback with regard to the heating of heating plates which have a large contact surface and are intended for heating large-surfaced workpieces.